Phase transitions of poly(L-lactic acid) in bulk and in solution.

摘要

Poly(L-lactic acid) (PLLA) has been subject to intensive research in recent years because of its biodegradability. Applications of PLLA, such as porous tissue engineering scaffolds and excipients for controlled release, have been developed in the biomedical and pharmaceutical fields. The crystal structures and morphologies of this semicrystalline polyester are the underlying factors that determine the mechanical and degradation properties of PLLA.;A fundamental understanding of the development of crystal structures and morphologies during the phase transitions of PLLA in bulk and in solution is pursued in this dissertation. Synchrotron wide- and small-angle x-ray scattering (WAXS and SAXS) are adopted as the primary experimental techniques. Crystallization during annealing and drawing in melt-spun PLLA fibers is studied by WAXS with a quantitative evaluation of crystallinity and orientation. This represents the study of phase transitions of PLLA in bulk at temperatures above glass transition temperature (Tg). It is found that chain orientation plays an important role in facilitating the re-crystallization of PLLA during annealing. Upon cooling, the densification of the amorphous phase of PLLA is observed by SAXS. The densification below T g of PLLA (63°C) is quantified by pycnometry measurements. A nonlinear dependence of the densities of the amorphous phase on temperature with a transition at about 45°C is found from pycnometry measurements. After thermal aging at this temperature, amorphous PLLA samples exhibit maximum excess enthalpies of relaxation. These observations are ascribed to the evolution of the amorphous phase of PLLA towards its equilibrium state via primary nucleation at temperatures below Tg.;The thermally induced phase separations (TIPS) of PLLA in solution are explored by SAXS, WAXS and scanning electron microscopy. A phase diagram of the PLLA/tetrahydrofuran (THF) binary system is established to explain the crystal structures and morphologies in the gels formed during phase separation. A new PLLA crystal form, the delta-form, is identified as a clathrate when PLLA/THF solutions are quenched to temperatures below 10°C. It is proposed that the delta-form crystal has a 4/1 helical chain conformation with THF molecules being incorporated in the interstitial sites of its unit cell. The crystallization kinetics during the gelation of PLLA solutions are studied by in situ simultaneous SAXS and WAXS. The Avrami parameters are evaluated from the SAXS invariant Q to determine the nucleation and growth behavior during the crystallization of PLLA in solutions. The dependence of the crystallization kinetics on gelation temperature and concentration is discussed.